Question What is space time?

Space is now considered a something and it is infused with time inseparable, hence space-time. It is expanding throughout the universe and it seems it is accelerating. [Surprisingly, Lemaitre's original paper (1927, IIRC) showed acceleration.] " Dark Energy" is used to help produce ideas on what is making it accelerate but none of the many models have made it to mainstream, as far as I know.
May 26, 2020
I believe space is a thing. Did it become something more than nothing, meaning does space now have mass?
I believe space is a thing. Did it become something more than nothing, meaning does space now have mass?
It's not what we would call normal like something we can touch and feel. Ancient times thought you kinda could and called it the Ether, and other names.

Since Dark Energy saturates it, so to speak, then it's just as if it has matter since energy and matter are equivalent. But DE is negative, so rather than attract space to itself, causing a collapsing effect, it pushes internally outward. DE became an idea in the late 90s to give this outward acceleration of space a label, but no one has any handle on it yet, though there are a couple dozen theories for it.
I understand space-time to be like a coordinate system. Am I misunderstanding?
A coordinate system is like a mapping plan. Variations in the density of matter (and energy) change the landscape from something, say, flat to regions with dimples, large and small. The analogy of putting a bowling ball on a trampoline is often used where the depression in the rubber sheet represents how the map, or coordinate system, can be used to properly take us from a simple, and incorrect, Cartesian X-Y grid and give us something that can help us produce correct results, such as acceleration rates through it. The spacing between the lines drawn represent the gradient -- the change in the rate of lines encountered with distance.

The mapping of the region around a small black hole, for instance, would reveal a severe gradient the closer one got to the EH. So, if someone were falling in this gradient would be so severe it would pull the body apart - spaghettification.

A GR physicist, however, I'm not, but this is my interpretation of the more basic view of it.